feat(笔记预览): 实现大文件分块加载功能

添加分块加载API接口及前端实现，支持大文件(>500KB)的分页加载，提升大文件预览体验
后端实现分块逻辑并添加权限检查，前端添加加载提示和滚动加载功能

biji-houdaun/src/main/java/com/test/bijihoudaun/controller/MarkdownController.java

@@ -3,6 +3,7 @@ package com.test.bijihoudaun.controller;
 import cn.hutool.core.util.ObjectUtil;
 import com.test.bijihoudaun.common.response.R;
 import com.test.bijihoudaun.entity.MarkdownFile;
+import com.test.bijihoudaun.entity.MarkdownFileChunk;
 import com.test.bijihoudaun.entity.MarkdownFileVO;
 import com.test.bijihoudaun.service.MarkdownFileService;
 import com.test.bijihoudaun.util.SecurityUtil;
@@ -117,4 +118,26 @@ public class MarkdownController {
         return R.success(files);
     }
 
+    @Operation(summary = "分块加载Markdown文件内容", description = "用于大文件（> 500KB）的分页加载")
+    @Parameters({
+        @Parameter(name = "id", description = "文件ID", required = true),
+        @Parameter(name = "chunkIndex", description = "块索引（从0开始）", required = false),
+        @Parameter(name = "chunkSize", description = "块大小（字符数），默认10000", required = false)
+    })
+    @GetMapping("/{id}/chunk")
+    public R<MarkdownFileChunk> getMarkdownChunk(
+            @PathVariable Long id,
+            @RequestParam(defaultValue = "0") int chunkIndex,
+            @RequestParam(defaultValue = "10000") int chunkSize) {
+        
+        // 获取当前认证状态
+        boolean isAuthenticated = SecurityUtil.isUserAuthenticated();
+        
+        MarkdownFileChunk chunk = markdownFileService.getMarkdownChunk(id, chunkIndex, chunkSize, isAuthenticated);
+        if (ObjectUtil.isNotNull(chunk)) {
+            return R.success(chunk);
+        }
+        return R.fail("文件未找到");
+    }
+
 }